Image transfer processes are well known in the art. These processes generally employ a single processing solution to develop an exposed image record and to produce a viewable image record.
Various formats for color diffusion transfer assemblages are described in the prior art such as U.S. Pat. Nos. 2,543,181, 2,983,606, 3,362,819, 3,362,821, 3,592,645, 3,785,815, 3,415,644, 3,415,645, 3,415,646, 3,647,437, 3,635,707 and 3,756,815 and Canadian Pat. Nos. 928,559 and 674,082. In these formats, the image-receiving layer containing the photographic image for viewing can be separated from the photographic layers after processing or, in some embodiments, when a transparent support is employed on the viewing side of the assemblage, it can remain permanently attached and integral with the image-generating and ancillary layers present in the structure. The image is formed by color-providing substances, released from the image-generating units, diffusing through the layers of the structure to the dye image-receiving layer. After exposure of the assemblage, an alkaline processing composition permeates the various layers to initiate development of the exposed photosensitive silver halide emulsion layers. The emulsion layers are developed in proportion to the extent of the respective exposures, and the image dyes which are formed or released in the respective image-generating layers begin to diffuse throughout the structure. At least a portion of the imagewise distribution of color-providing substances diffuse to the dye image-receiving layer to form an image of the original subject.
It is common in the art to provide the image generating layer on one element with a so-called cover sheet as another element. The image generating element and the cover sheet are then placed together to form a laminate either at the time of manufacture for integral film units or after the exposure of the image generating layer. After exposure, an alkaline processing composition can be discharged between the cover sheet and the image generating element.
The cover sheet for the image transfer film unit can have a wide variety of layers that serve various functions. One combination of layers that is commonly found in the cover sheet is an acid neutralizing layer in conjunction with a timing layer. The alkaline processing composition penetrates through the timing layer and alkali is depleted in the film unit by the acid in the neutralizing layer. Depletion of the alkali serves several functions, the most important of which is usually the immobilization of the dyes. The cover sheet can also have other layers such as subbing layers, light filter layers, curl control layers, friction reducing layers and the like depending on the particular format of the image transfer film unit.
The timing layer of the image transfer film unit serves to delay the release of acid from the neutralizing layer for a predetermined period. The timing layer may be an inert spacer layer in which case the delay results principally from the time required for the alkali to physically pass through the layer. Inert spacer timing layers, even when very thick, only provide for a short delay. Examples of inert spacer layers are layers of gelatine, poly(vinyl alcohol), carboxymethylcellulose, polyacrylamide, hydroxypropylcellulose and the like. Alternatively, the timing layer may be a barrier timing layer in which case the delay results not only because of the time required for physical permeation but principally because of the time required for chemical reaction. A barrier timing layer is initially substantially impermeable and time is required to allow the aqueous alkaline solution to react with the layer and increase its permeability. Usually the permeability increasing reaction that takes place is the hydrolysis or neutralization of the layer by the alkaline solution. Typically, therefore, a barrier timing layer comprises a substantially alkaline solution impermeable material which can be hydrolyzed or neutralized by the alkaline solution to a substantially alkaline solution permeable material. Examples of barrier timing layers include the timing layers described in Ser. No. 676,945 filed Apr. 14, 1976 by Hannie now U.S. Pat. No. 4,056,394; layers composed primarily of cellulose acetate having an acetate content of about 40 percent such as described in U.S. application Ser. No. 521,221 filed Nov. 5, 1974 by Abel now U.S. Pat. No. 4,009,030. Other barrier timing layers include polyvinyl acetate or mixtures thereof, copolymers of dimethoxymethylene such as described in German OLS No. 2,455,762 and the like.
In some image transfer processes, the image generating element also contains the image receiving layer. In these formats the image generating element can be described as an integral imaging receiver element. After imagewise exposure and during processing, the generated image diffuses to the image receiving layer. In some embodiments of this process the image may be formed by exposure of the image generating element through a transparent cover sheet. A hybrid type of image transfer process is described in U.S. Pat. No. 3,836,365. In this process both the image-generating element and the cover sheet have an image receiving layer. With one exposure, two images can be formed in the two image receiving layers. Unfortunately, the image formed in the integral-generating element is commercially unsatisfactory. Thus, the problem of obtaining satisfactory images in the image generating element and at the same time images which have good physical properties, has not been solved.
It is desirable, in order to provide for processing of the element in ambient room light conditions, that the viscous processing composition that is discharged between the transparent cover sheet and the integral imaging receiver be opaque so as to prevent the fogging of the integral imaging receiver during processing. It will be readily apparent that any discontinuities in the opaque processing composition will produce undesirable areas of maximum density in the final image due to the fogging of the image generating element through the discontinuity. The fogging may either be caused because of light exposing the integral imaging receiver or because the alkali can not be neutralized by the acid from the cover sheet in the area of the discontinuity. In the latter case the dark spot is caused by overdevelopment. Whatever the mechanism, the discontinuities in the processing composition produce what we refer to herein as dark spots.
Unfortunately, it has been found that when the viscous processing composition is discharged between the cover sheet and the integral imaging receiver, bubbles are frequently entrained. Generally, the bubbles that are entrained are small enough so as not to form a discontinuity in the processing composition. However, it has been found that a large number of these relatively small bubbles coalesce during the first few minutes after the processing composition is discharged. These bubbles can coalesce to a size which results in a discontinuity being formed in the processing composition. In the majority of film units, the picture area is defined by a black aperture mask placed between the cover sheet and image generating element. Coalesced bubbles have a tendency to collect at the edges of the aperture and the resultant dark spots have therefore been designated aperture border imperfections. A lesser number of coalesced bubbles may cause discontinuities in the processing composition and resultant dark spots within the aperture area. Further, it has been found that in some instances user handling of the film unit during these first critical minutes or structural stresses in the film unit can cause delamination of the cover sheet from the image generating element. This delamination can also cause discontinuities in the processing composition in the form of finger like projections from the edges of the film unit. In film units having a timing layer which is a barrier layer, bubble formation and delamination are particularly evident.
While the effect of bubble formation and delamination has been described with particular reference to a process wherein the integral imaging receiver can be fogged due to a discontinuity in the processing composition, it is readily apparent that any nonuniformity in the processing composition could be detrimental to the final image regardless of the format.
The prior art has not recognized that the small bubbles formed in the processing composition when it is discharged between the cover sheet and the image generating element may coalesce to form larger bubbles which may form discontinuities in the processing composition. The prior art also lacks a solution to the problem of delamination of the cover sheet from the image generating element during processing.